Method and apparatus for controlling a patient&#39;s body temperature by in situ blood temperature modification

ABSTRACT

The present invention provides a method and apparatus for controlling the internal body temperature of a patient. According to the present invention, a catheter is inserted through an incision into a large blood vessel of a patient. By selectively heating or cooling a portion of the catheter lying within the blood vessel, heat may be transferred to or from blood flowing within the vessel and the patient&#39;s body temperature may thereby be increased or decreased as desired. The invention will find use in treating undesirable conditions of hypothermia and hyperthermia, or for inducing a condition of artificial hypothermia when desired. The method and system further provide for the cooling of initially hypothermic patients whose blood or body temperature has been warmed above the desired target level and the warming of initially hyperthermic patients whose blood or body temperature has been cooled below the desired target temperature.

RELATED APPLICATIONS

This patent application is a continuation of copending U.S. patentapplication Ser. No. 11/044,997 filed Jan. 26, 2005, which is acontinuation of U.S. patent application Ser. No. 10/643,321 filed Aug.19, 2003 and now issued as U.S. Pat. No. 6,849,083, which is acontinuation of U.S. patent application Ser. No. 10/004,579 filed Dec.4, 2001 and now issued as U.S. Pat. No. 6,635,076 which is acontinuation of U.S. patent application Ser. No. 09/522,135 filed onMar. 9, 2000 and now issued as U.S. Pat. No. 6,436,131 which is acontinuation of U.S. patent application Ser. No. 09/131,081 filed Aug.7, 1998 and now issued as U.S. Pat. No. 6,149,676 which is a division ofSer. No. 08/584,013 filed Jan. 8, 1996 and now issued as U.S. Pat. No.5,837,033, the entire disclosures of which are incorporated herein byreference. This application does not claim priority prior to Jan. 8,1996.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to the selective modificationand control of a patient's body temperature. More particularly, thepresent invention provides methods and apparatus for treatinghypothermia or hyperthermia by inserting a catheter into a blood vesselof the patient and selectively transferring heat to or from bloodflowing through the vessel.

2. Description of the Background Art

Under ordinary circumstances the thermoregulatory system of the humanbody maintains a near constant temperature of about 37 .degree. C. (98.6.degree. F.). Heat lost to the environment is precisely balanced by heatproduced within the body.

Hypothermia is a condition of abnormally low body temperature.Hypothermia can be clinically defined as a core body temperature of 35degrees C. or less. Hypothermia is sometimes characterized furtheraccording to its severity. A body core temperature in the range from 32degrees C. to 35 degrees C. is described as “mild” hypothermia, 30degrees C. to 32 degrees C. is called “moderate,” 24 degrees C. to 30degrees C. is described as “severe,” and a body temperature less than 24degrees C. constitutes “profound” hypothermia. Although the above rangesprovide a useful basis for discussion, they are not absolutes anddefinitions vary widely in the medical literature.

Accidental hypothermia results when heat loss to the environment exceedsthe body's ability to produce heat internally. In many cases,thermoregulation and heat production are normal but the patient becomeshypothermic due to overwhelming environmental cold stress. This is arelatively common condition, often resulting from exposure to theelements. Hypothermia may also occur in patients exposed to mild coldstress whose thermoregulatory ability has been lessened due to injury orillness. For example, this type of hypothermia sometimes occurs inpatients suffering from trauma or as a complication in patientsundergoing surgery.

Hypothermia of either type is a dangerous condition which can haveserious medical consequences. In particular, hypothermia interferes withthe ability of the heart to pump blood. Hypothermia may be fatal forthis reason alone. Additionally, low body temperature seriouslyinterferes with the enzymatic reactions necessary for blood clotting.This sometimes results in bleeding that is very difficult to control,even when normal clotting factor levels are present. These effects andother adverse consequences of hypothermia lead to drastically increasedmortality rates both among victims of trauma and in patients undergoingsurgery.

Simple methods for treating hypothermia have been known since very earlytimes. Such methods include wrapping the patient in blankets,administering warm fluids by mouth, and immersing the patient in a warmwater bath. While these methods are very effective for mild hypothermia,more intrusive methods have been developed for treating severe andprofound cases of hypothermia. In particular, methods have been devisedto effect direct heating of a patient's blood. Most commonly, blood iswithdrawn from patient's circulation, passed through external warmingequipment, and reinfused back into the patient. Alternatively, the useof heated catheters has been proposed, where a catheter having a heatingelement near its distal end is inserted into the patient's vasculatureand heat directly transferred into the patient's circulating blood.

While the direct heating of patient blood can be highly effective, evenin treating severe and profound cases of hypothermia, it has beenobserved by the inventor herein that the excess transfer of heat cancause the patient's temperature to rise above normal body temperature,resulting in hyperthermia. Hyperthermia can occur, for example, when ahypothermic patient's metabolism begins to produce substantial amountsof heat at the same time heat is being transferred directly to theblood.

It would therefore be desirable to provide methods for treatinghypothermia which further provide for treatment of accidental orincidental hyperthermia. In particular, it would be desirable to developsystems and methods for transferring heat to the blood where heat can beoptionally removed if the patient blood or body temperature exceeds atarget level. Such methods and devices will preferably employ a catheterfor direct heat transfer into circulating blood, but could also beuseful with methods where blood is heated externally from the patient'sbody. Such systems and methods should further be useful for thetreatment of patients who are initially hyperthermic, where the methodsand systems provide for initial cooling of the blood and optionalheating of the blood should the patient blood or body temperature fallbelow a target temperature.

SUMMARY OF THE INVENTION

The present invention provides apparatus and methods for restoringnormal body temperature in patients initially suffering from hypothermiaor hyperthermia. The apparatus includes a catheter and a control unitwhich together permit selective heating and cooling of the patient'scirculating blood. For hypothermic patients, the method will provide forinitially heating the blood until a target blood or body temperature hasbeen restored. Heating will be stopped after reaching the targettemperature. Even after the heating has been stopped, however, thepatient's blood and/or body temperature will continue to be monitored toassure that the blood or body temperature does not overshoot the target.As discussed above, an initially hypothermic patient can becomehyperthermic if the total amount of heat experienced from both patientmetabolism and external heating exceeds that necessary to restore normalbody temperature. In the case of patients entering hyperthermia, themethod of the present invention provides for cooling the patient'sblood, usually using the same intravascular catheter or other apparatuswhich has been used for heating.

In the case of initially hyperthermic patients, the method of thepresent invention relies on cooling the patient's blood in order toreduce the blood and body temperature. Cooling will stop after a targettemperature has been reached. The patient's blood and/or bodytemperature will continue to be monitored, however, and should thepatient enter hypothermia, normal body temperature can then be restoredby introducing an appropriate amount of heat to the circulating blood.

According to a first aspect of the present invention, a system forrestoring normal body temperature to a patient comprises anintravascular catheter having at least one heat transfer surface, atemperature sensor, and a control unit connectable to the temperaturesensor and the catheter. The control unit selectively transfers heat toor from the at least one heat transfer surface in order to achieve adesired target blood or body temperature. The intravascular catheter maycomprise a single heat transfer surface for both heat generating andheat absorption, but will usually comprise both a heat-generatingsurface and a separate heat-absorbing surface. The heat-generatingsurface will typically comprise a resistance heater, such as a wirecoil, and the heat-absorbing surface will typically comprise a metalfoil wrapped around the catheter, typically having an exposed area of atleast about 2 cm². In such cases, the control unit may comprise anelectrical current source connectable to the resistance heater and athermal electric cooler connectable to the metal foil. In an alternativeconstruction, the catheter may include at least one flow lumen whichpermits flow of a heat exchange medium within the catheter past the heattransfer surface. The control unit will then include a heater, a cooler,and a controller for selectively activating the heater or cooler totransfer heat to the heat exchange medium in order to restore normalbody temperature to the patient. The heater may be an electricalresistance heater and the cooler may be a thermoelectric cooler.

The temperature sensor will typically be on the catheter and measure theblood temperature. Alternatively or additionally, temperature sensor(s)may be separately attachable to the patient to measure body temperature.

In a second aspect of the present invention, a catheter for restoringnormal body temperature to a patient by selectively transferring heat toor from a patient's blood flow comprises a catheter body having aproximal end and a distal end. The distal end is insertable into a bloodvessel, and the heat-generating heat exchange surface and aheat-absorbing heat exchange surface are both disposed near the distalend of the catheter body. Typically, the catheter body will have alength in the range from about 15 cm to 50 cm and a diameter in therange from 1 mm to 5 mm. The heat-generating heat transfer surface willusually comprise an electrical resistance heater, and the catheter willfurther comprise a connector which connects the resistance heater to anexternal current source. The heat-absorbing heat transfer surface willtypically comprise a metal foil wrapped around the catheter body, and aheat-conductive element will extend through the catheter body to nearthe proximal end to permit the heat-absorbing foil to be connected to acooler in a separate control unit. The metal foil heat-absorbing surfacewill typically have an area of at least 2 cm², usually being from 4 cm²to 80 cm². The heat-conductive element could be either a continuation ofthe metal foil surface (preferably being insulated in portions whichwill not lie within the blood circulation), or alternatively could be ametal core composed of a heat-conductive material.

According to the method of the present invention, normal bodytemperatures are restored to a patient by selectively introducing heatto the patient's blood flow for hypothermic patients or removing heatfrom the blood flow for hyperthermic patients. Usually, the heat will beintroduced or removed via an intravascular catheter which is connectedto an external control unit. Alternatively, the method of the presentinvention will also comprise the direct extracorporeal heating andcooling of the blood. A temperature characteristic of the patient ismonitored, typically being blood temperature and/or body temperature. Ifthe temperature characteristic indicates that initially hypothermicpatients have or are about to become hyperthermic, then heat will beremoved from the circulating blood to restore normal body temperature.Similarly, if the monitored temperature characteristic indicates thatinitially hypothermic patients are about to become hyperthermic, thenheat will be removed from the blood of those patients until normal bodytemperature has been restored.

The preferred intravascular catheters will be inserted into a bloodvessel, usually being the femoral artery or vein, or the jugular arteryor vein. The heat-introducing step comprises introducing heat at a ratebetween 10 W and 500 W, usually between 50 W and 250 W, while the heatremoving step comprises removing heat at a rate from 1 W to 100 W.Preferably, the catheter and system described above will be employed.

For initially hypothermic patients, the temperature characteristic willusually be blood temperature, and the target blood temperature, i.e.,temperature at which heating is stopped, will be 36.9 degrees C. Shouldthe blood temperature exceed 39 degrees C., then cooling will commence.For initially hyperthermic patients, the preferred temperaturecharacteristic will be blood temperature, and the target temperature atwhich cooling will be stopped will be about 36.9 degrees C. Should theblood continue to cool, typically to a temperature of 36 degrees C. orbelow, then blood heating will commence. Is should be appreciated,however, that these temperature targets are nominal objectives, and themethods of the present invention can be practiced with targettemperatures which differ somewhat from those just set forth.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a catheter according to the present invention insertedpercutaneously into a blood vessel of a patient;

FIG. 2 depicts a catheter suitable for increasing the temperature of apatient's blood by electrical resistance heating;

FIG. 3 depicts the distal end of a catheter having a resistance heatingelement and a temperature sensor;

FIG. 4 depicts the distal end of a catheter having an optical wave guideand an optical diffusing tip for converting laser energy into heat;

FIG. 5 depicts a catheter in which heat is transferred down a thermallyconductive shaft between the distal end of the catheter and heating orcooling apparatus at the proximal end of the shaft;

FIG. 6 depicts a catheter in which a heated or cooled fluid flowsthrough a balloon, which provides for an increased surface area at thedistal end;

FIG. 7 depicts a catheter having a resistance heating element at itsdistal end and a balloon having longitudinal ribs to further increasethe heat transfer surface area;

FIG. 8A depicts a catheter having longitudinal fins at the distal end ofthe catheter body;

FIG. 8B depicts a catheter having radial ribs at the distal end of thecatheter body; and

FIG. 8C depicts a catheter having a spiral fin to increase the heattransfer area at the distal end of the catheter.

FIG. 9 illustrates a catheter having a resistance heater which heats afluid filling a balloon. Current flows through the fluid from a pair ofconduction wires embedded in the catheter body.

FIG. 10 illustrates the control schemes for raising body temperature ina patient suffering from hypothermia and lowering body temperature in apatient suffering from hyperthermia, respectively.

FIG. 11 illustrates a preferred catheter for the selective heating andcooling of patient blood flow employing a wire coil resistance heaterand a metal foil cooling element.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention provides methods and apparatus for selectivelymodifying and controlling a patient's body temperature by warming orcooling the patient's blood, usually using an intravascular catheter insitu. According to the preferred method of the present invention, thecatheter is inserted through a puncture or incision into a blood vesselin the patient's body. By warming or cooling a portion of the catheter,heat may be transferred to or from blood flowing within the vessel andthe patient's body temperature may thereby be increased or decreased asdesired. During the procedure, the patient's blood and/or body coretemperature may be independently monitored and treatment may continueuntil the patient's blood and/or body core temperature approaches thedesired level, usually the normal body temperature of about 37 degreesC. Such methods will find use in treating undesirable conditions ofhypothermia and hyperthermia and may also be used to induce anartificial condition of hypothermia when desired, e.g., to temporarilyreduce a patient's need for oxygen. In such a case, the patient'stemperature may be reduced several degrees Celsius below the normal bodytemperature.

In treating conditions of hypothermia and hyperthermia there is thepossibility that the patient's core body temperature will “overshoot”the target normal body temperature. The body's metabolic response to theexternal heating or cooling being applied, as described above, canresult in overcompensation of the initial condition. In particular, whenheating the patient's body to treat hypothermia, the body's own heatgeneration arising from internal metabolic processes may raise the bodytemperature in an unpredictable manner, resulting in a body temperaturethat can rise well above normal body temperature. In such cases, thepresent invention provides for a reversal of the transfer of heat fromor to the patient's blood. In the case of an uncontrolled temperaturerise, the system of the present invention will be switched so that heatwill be withdrawn from the circulating blood. Conversely, in the case ofovercooling of the patient's body, the system will be switched so thatheat will be introduced to the patient.

FIG. 1 depicts a distal end 15 of a catheter 10 according to the presentinvention. The catheter has been inserted through the patient's skininto a blood vessel BV. Blood flow through the vessel is indicated by aset of flow arrows F. Preferably, the catheter will be inserted into arelatively large blood vessel, e.g., the femoral artery or vein or thejugular vein. Use of these vessels is advantageous in that they arereadily accessible, provide safe and convenient insertion sites, andhave relatively large volumes of blood flowing through them. In general,large blood flow rates facilitate quicker heat transfer into or out ofthe patient.

For example, the jugular vein may have a diameter of about 22 French, ora bit more than 7 millimeters (1 French=0.013 inches=0.33 mm). Acatheter suitable for insertion into a vessel of this size can be madequite large relative to catheters intended for insertion into otherregions of the vascular system. Atherectomy or balloon angioplastycatheters are sometimes used to clear blockages from the coronary arteryand similar vessels. These catheters commonly have external diameters inthe range between 2 and 8 French.

In contrast, it is anticipated that a catheter according to the presentinvention will typically have an external diameter of about 10 French ormore, although this dimension may obviously be varied a great dealwithout departing from the basic principles of the claimed invention. Itis desirable that the catheter be small enough so that the puncture sitecan be entered using the percutaneous Seldinger technique, a techniquewell known to medical practitioners. To avoid vessel trauma, thecatheter will usually be less than 12 French in diameter upon insertion.Once in the vessel however, the distal or working end of the cathetercan be expanded to any size so long as blood flow is not unduly impeded.

Additionally, the femoral artery and vein and the jugular vein are allrelatively long and straight blood vessels. This will allow for theconvenient insertion of a catheter having a temperature controlledregion of considerable length. This is of course advantageous in thatmore heat may be transferred at a given temperature for a catheter of agiven diameter if the length of the heat transfer region is increased.

Techniques for inserting catheters into the above mentioned bloodvessels are well known among medical personnel. Although the method ofthe present invention will probably be most commonly employed in ahospital, the procedure need not be performed in an operating room. Theapparatus and procedure are so simple that the catheter may be insertedand treatment may begin in some cases even in an ambulance or in thefield.

The distal end 15 of the catheter may be heated or cooled as desired andheld at a temperature either somewhat above or somewhat below thepatient's body temperature. Blood flowing through the vessel willthereby be warmed or cooled. That blood will be circulated rapidlythroughout the patient's circulatory system. The beneficial effect ofwarming or cooling the patient's blood in the vicinity of the catheterwill thereby be spread very quickly throughout the entire body of thepatient.

FIGS. 2 and 3 depict a catheter suitable for treating hypothermia byincreasing the temperature of a patient's blood. As depicted in FIG. 2,the catheter has a preferably flexible catheter body 20. Disposed withinthe catheter body are a pair of electrical conduction leads 22 and 23and a temperature measurement lead 25.

Electrical conduction leads 22 and 23 are connected to a resistanceheating element 28, as depicted in FIG. 3. Electrical current providedby a power source (not shown) is converted to heat within the heatingcoil. That heat warms distal end 15 of the catheter and is therebytransferred to blood flowing through the vessel.

Temperature measurement lead 25 is connected to a temperature sensor 30.The temperature sensor facilitates the control of current flow throughthe heating coil. It is important to closely monitor the temperature ofthe distal end of the catheter and thus the flow of heat into thepatient's blood. Care must be taken not to overheat the blood whilestill providing an adequate rate of heat transfer into the patient. Theprovision of a sensitive temperature sensor at the distal end of thecatheter will help to achieve this goal.

FIG. 4 depicts an alternate embodiment of a catheter having means fortransferring energy from an external power source to distal end 15 ofcatheter body 20. In this embodiment, laser energy from a laser lightsource (not shown) is transmitted along optical wave guide 35. The waveguide directs the laser energy into optical diffusing tip 37, whichconverts the laser energy to heat. From diffusing tip 37, the heatradiates outward into distal end 15 of the catheter and from there intothe patient's blood stream.

FIG. 5 depicts another catheter suitable for practicing the presentinvention. This embodiment has a thermally conductive shaft 40 runningthe length of catheter body 20. Shaft 40 is made of a metal or othermaterial having a high thermal conductivity. By heating or cooling theproximal end 42 of shaft 40 with an external heating or coolingapparatus 45, heat will be caused to flow either into or out of thedistal end 47 of the shaft. In the embodiment depicted, the distal endof the shaft is fitted with heat transfer vanes 50, which add to thesurface area of the shaft and thereby promote more effective heattransfer between the catheter and the patient's blood stream.

FIG. 6 depicts still another means for transferring heat to or from thedistal end of a catheter. In this embodiment, catheter body 20 has twolumens running through it. Fluid flows from the proximal end of thecatheter through in-flow lumen 60, through a heat transfer region 62,and back out through out-flow lumen 64. By supplying either warmed orcooled fluid through inflow lumen 60, heat may be transferred either toor from the patient's blood stream.

In the embodiment depicted, heat transfer region 62 is in the form of aballoon 70. Use of a balloon will be advantageous in some embodiments toprovide an increased surface area through which heat transfer may takeplace. Balloon inflation is maintained by a pressure difference in thefluid as it flows through in-flow lumen 60 and out-flow lumen 64. Theballoon should be inflated to a diameter somewhat less than that of theinside diameter of the blood vessel so as not to unduly impede the flowof blood through the vessel.

FIG. 7 depicts a catheter having an internal resistance heating element28 and a balloon 70, which is shown inflated. In this embodiment, theincreased surface area provided by the inflated balloon is furtheraugmented by the presence of a set of longitudinal fins 75 on thesurface of the balloon. Alternatively, longitudinal fins 75, radial ribs77, or one or more spiral fins 79 may be disposed directly on the body20 of a catheter as shown in FIGS. 8A, 8B and 8C. Ordinarily,longitudinal ribs will be most advantageous because they restrict bloodflow through the vessel less than other configurations. In fact, theseribs insure that the balloon will not block the flow of blood throughthe vessel because a flow path will always be maintained (between theribs) regardless of how much the balloon is inflated.

Inclusion of a balloon on a catheter employing resistance heating allowsfor designs in which current is conducted through the fluid which fillsthe balloon. The catheter depicted in FIG. 9 has a catheter body 20about which is disposed an inflatable balloon 70. The balloon isinflated by injecting a suitable fluid into the balloon through centralballoon inflation lumen 80. In this embodiment, current flows from anexternal source of electrical power (not shown) through conduction wires82 and 84 to electrodes 86 and 88.

A suitable fluid will allow current to flow between electrodes 86 and88. Common saline solution, for example, contains dissolved ions whichcan serve as charge conductors. Electrical resistance within the fluidwill cause the fluid to be heated, thus providing the desired warming ofthe catheter. The amount of warming will be dependant upon the voltagebetween the electrodes, the distance between them, and the resistivityof the fluid. The relation between these quantities is fairly simple;one skilled in the art will have no difficulty selecting appropriatevalues.

Resistance heating catheters like those depicted in FIGS. 3, 7 and 9 mayuse DC or low frequency AC power supplies. However, it may be desirableto use a higher frequency power supply. For example, it is known thatthe risk of adverse physiological response or electrocution response maybe lessened at frequencies within the range of about 100 kilohertz to 1megahertz. Power supplies that operate at these frequencies are commonlyreferred to as radio-frequency, or RF, power supplies.

A catheter according to the present invention should be designed tooptimize the rate of heat transfer between the catheter and bloodflowing through the vessel. While a large surface area is desirable inorder to maximize heat transfer, care must be taken so that the catheterdoes not unduly restrict blood flow through the vessel. Furthermore, thetemperature of the catheter should be carefully controlled to preventundesirable chemical changes within the blood. This is especiallyimportant when applying heat to the blood as blood is readily denaturedby even moderately high temperatures. The exterior temperature of acatheter for warming blood should generally not exceed about 42 .degree.C.-43 .degree. C.

It is estimated that a catheter whose surface temperature is controlledbetween 37 degrees C. and 42 degrees C. will provide a body core warmingrate of approximately one to two degrees Celsius per hour in a patientstarting out with severe hypothermia. This estimate is highly dependanton a number of factors including the rate of blood flow through thevessel, the initial body temperature of the patient, the externalsurface area of the catheter through which heat is conducted, etc. Theactual rate achieved may vary substantially from the above estimate.

The above estimate provides a starting point for a rough estimate as tothe level of power transferred from the catheter to the patient's bodyand therefore of the size of the power supply required by the system.Regardless of the exact means of power transmission chosen, resistanceheating coil, laser and diffusing tip, direct conduction or fluidcirculation, an appropriate power supply will be required to provideheat to the system.

The sum of heat entering and leaving a patient's body can be written as:

.DELTA.H=H.sub.c+H.sub.i−H.sub.e

where .DELTA.H is the sum of all heat transferred, H.sub.c is the heattransferred from the catheter to the patient, H.sub.i the heat producedby the patient internally, and He the heat lost from the patient to theenvironment. If one assumes, as will ordinarily be the case in a healthypatient, that the body's internal thermoregulatory system will producejust enough heat to offset heat lost to the environment, then theequation is made simple:

.DELTA.H=H.sub.c.

The above equation can be written in terms of the change in thepatient's internal body temperature over time as follows:

mc(.DELTA.T/.DELTA.t)=(.DELTA.H.sub.c/.DELTA.t)

where m is the body mass of the patient, c is the specific heat of thepatient's body, (.DELTA.T/.DELTA.t) is the time rate of change of thepatient's internal body temperature, (.DELTA.H.sub.c/.DELTA.t) is thetime rate of heat delivery from the catheter to the patient.

If one assumes a patient having a body mass of 75 kilograms and aspecific heat of 4186 joules/.degree. C.-kg (assumes the specific heatof the human body to be the same as that of water, the actual value willbe somewhat different), then a warming rate of 1 .degree. C. per hour(3600 seconds) will require the catheter to transfer heat to the patientat a rate of about 87 watts (1 watt=1 joule/sec).

However, as an estimate of the desirable size of a power supply to beused with a catheter of the present invention, this estimate is almostcertainly too low. This is true for a number of reasons. First, it wasassumed for the sake of convenience that the patient's internal systemwould produce an amount of heat equal to that lost to the environment.In a hypothermic patient this will obviously not be the case. Almost bydefinition, hypothermia occurs when a person's ability to produce heatinternally is overwhelmed by heat lost to the environment. The catheterwill have to make up the difference so the power level required willneed to be greater for that reason alone.

Additionally, the above estimate does not allow for power losses betweenthe power supply and whatever warming means is utilized. Such lossescould include resistance losses in electrical transmission lines betweenthe power supply and a resistance heating element, inherentinefficiencies and other losses in a system having a laser and adiffusing tip, heat losses along a thermally conductive shaft or fluidcirculation lumen, and the like. Any such losses which do occur willneed to be compensated for by additional power supply capacity.

Furthermore, it would be undesirable to limit the performance of acatheter according to the present invention by limiting the size of thepower supply used. It would be preferable instead to use a power supplycapable of providing power considerably in excess of that actuallyneeded and then controlling the delivery of that power according to themeasured temperature of the catheter itself. As mentioned previously,this can be readily accomplished by including a sensitive temperaturesensor within the body of the catheter. Nevertheless, the abovecalculation can be used as a useful estimate of the likely lower boundfor sizing a power supply for use in a catheter according to the presentinvention.

An alternative estimate can be made by comparing the likely performanceof the various embodiments described herein with the power requirementsfor the external blood warming apparatus presently known. Such externalwarming apparatus generally requires a supply of power on the order of1000-1500 watts and sometimes more. A device according to the presentinvention will most likely require considerably less power than that.First, the present invention requires no external pump to circulate theblood; this function is provided by the patient's own heart.Accordingly, no power is needed to drive such a pump. Secondly, thepresent invention is considerably less complicated than external bloodwarming systems. Known systems circulate the blood over a relativelylengthy path from the patient, through the warming element, and backinto the patient. It is expected that more heat is lost over thislengthy path than will be lost in any device according to the presentinvention.

Thus, the power required by external blood circulation and warmingsystems of the type previously known can be used as a rough estimate ofthe likely upper limit for power required by a system according to thepresent invention. It is most likely that such a system will best beequipped with a power supply having a capacity somewhere between the tworough estimates described above. It is therefore contemplated that asuitable power supply will be capable of providing peak power somewherein the range between 100 and 1500 watts, probably being in the rangebetween 300 and 1000 watts. The ranges specified are an estimate ofsuitable peak power capability. The power supply will most commonly bethermostatically controlled in response to a temperature sensor in thebody of the catheter. The actual effective power transmitted to thepatient will therefore typically be much less than the peak powercapacity of the system power supply.

With respect to a catheter for cooling, the temperature and powerconstraints are not as limiting as is the case in a catheter for warmingblood. Care should merely be taken to avoid freezing the blood orinducing shock to the patient from too rapid cooling.

Blood is essentially water containing a number of suspended anddissolved substances. As such, its freezing point is somewhat below 0.degree. C. However, a catheter adapted to cool blood in a hyperthermicpatient or to induce an artificial hypothermia will usually not beoperated at temperatures that low. It is presently contemplated that theexternal surface of such a catheter may be held in the range betweenabout 20 .degree. C. and 24 .degree. C., although the actual temperaturecould vary between about 0 .degree. C. and the patient's current bodytemperature (somewhat in excess of 37 .degree. C.).

Various embodiments of apparatus suitable for practicing the methods ofthe present invention have been described. Other embodiments andmodifications will occur to those skilled in the art. For example,various means for heat transfer, e.g., resistance, including radiofrequency, heating; laser energy; pumped fluids; etc., may be combinedwith various means for increasing the effective heat transfer surfacearea, e.g., balloons, fins, ribs, etc., to optimize the function of adevice according to the present invention. Also, a temperature sensorwill typically be used although for ease of illustration such a sensoris not depicted in all of the embodiments described. Furthermore,although most of the figures depict embodiments in which only a limitedportion of the catheter is temperature controlled, no reason exists toprevent warming or cooling substantially the whole length of thecatheter.

Broadly stated, the present invention provides a method for modifying apatient's body temperature by controlling the temperature of a catheterinserted into a blood vessel of the patient. Although severalillustrative examples of means for practicing the invention aredescribed above, these examples are by no means exhaustive of allpossible means for practicing the invention. The scope of the inventionshould therefore be determined with reference to the appended claims,along with the full range of equivalents to which those claims areentitled.

The present invention thus provides methods for both raising the bodytemperature of initially hypothermic patients and lowering the bodytemperature of patients who are initially hyperthermic or for whom thebody temperature is to be lowered below normal for some other purpose.In all cases, it is possible that the target body temperature will beinadvertently exceeded due to an uncontrollable physiologic response ofthe patient, e.g., initially hypothermic patients may becomehyperthermic and initially hyperthermic patients may become hypothermic.In such cases, the present invention specifically provides for reversingthe heat transfer process so that patients passing into hyperthermia canbe immediately cooled and patients passing into hypothermia can beimmediately warmed. The control schemes for both warming initiallyhypothermic and cooling initially hyperthermic patients are set forth inFIG. 10. The initial, target, and overshoot temperatures for bothinitially hyperthermic and initially hypothermic patients are set forthin Table 1 below.

A preferred system for the selective warming and cooling of patients isillustrated in FIG. 11. The system comprises a catheter 100 having aproximal end 102, a distal end 104, a heat-generating surface 106 nearthe distal end, and a heat-absorbing surface near the distal end 108.The heat-generating surface 106 can be any of the heat transfercomponents described above, but will preferably be a wire coilresistance heater having from 50 to 1000 windings, typicallyspaced-apart from 0.1 mm to 1 mm. The total length of the catheter willtypically be from 15 cm to 50 cm, and the diameter will be from 1 mm to5 mm. Usually, the windings will extend over a total distance in therange from 10 cm to 20 cm near the distal end.

The exemplary heat-absorbing surface will be a thermally conductivemetal foil, typically composed of a biologically compatible thermallyconductive metal, such as gold, silver, aluminum, or the like. Copperwill also be useful, but will have to be treated or encapsulated inorder to enhance biocompatibility. The foil will typically be thin inorder to enhance flexibility of the catheter body, typically having athickness in the range from 0.001 mm to 0.01 mm.

The heat-absorbing surface 108 will be conductively coupled to a coolerlocated externally of the catheter, typically in a control unit 120 asdescribed below. in the illustrated embodiment, the surface 108 iscoupled by a thermally conductive core member 110 composed of a flexiblerod or wire formed from one of the thermally conductive metals describedabove. Alternatively, thermal coupling can be achieved by extending thesurface 108 proximally so that the proximal end of the surface can becoupled to the cooler. In the latter case, it will be preferable thatthe proximal portions of the surface 108 be thermally insulated toprevent cooling outside of the blood circulation.

The system will further comprise a control unit 120 which typicallyprovides both the heat-generator and the cooler for coupling to thecatheter 100. The heat-generator will usually comprise a direct currentsource for coupling to the resistance heater on the catheter. Usually,the direct current source will be a commercially available,temperature-controlled DC power supply, typically operating at a voltagein the range from 10 VDC to 60 VDC and a current output in the rangefrom 1 A to 2.5 A. Usually, the power supply will be controlled tomaintain the surface temperature of the heating surface 106 in the rangefrom 40 degrees C. to 42 degrees C. As discussed above, the surfacetemperature should not exceed 42 degrees C. in order to prevent damageto blood components. Other desirable characteristics of the heatexchange surface are described above.

Optionally, the temperature of the heat exchange surface can also becontrolled based on measured blood temperature and/or measured bodytemperature. Blood temperature can be measured by temperature sensorspresent on the catheter. For example, a temperature sensor 112 may belocated on the catheter spaced-apart from the heat exchange surfaces 106and 108. The temperature sensor 112 may be located either upstream ordownstream from the heat exchange surfaces based on the direction ofblood flow and depending on the manner in which the catheter isintroduced to the patient. Optionally, a pair of temperature sensorscould be provided, one disposed on each side of the heat exchangesurfaces in order to measure both upstream and downstream bloodtemperatures. The catheter will also include a temperature sensor (notillustrated) coupled directly to the heat-generating surface 106 so thatthe temperature of the surface may be directly controlled. Othertemperature sensors (not illustrated) may be provided for directlymeasuring the patient's core body temperature, with the core bodytemperatures being fed back into the control unit 120.

The cooler in control unit 120 may be any type of refrigeration unitcapable of removing heat from the heat-absorbing surface 106 at a ratesufficient to cool the blood at a desired rate. Typically, the coolerwill be rated at from 1 W to 100 W. Preferably, the cooler will be athermoelectric cooler, such as those commercially available from MelcorThermoelectrics, Trenton, N.J. 08648. The cooler will be directlycoupled to the core element 110 so that direct heat conduction from theheat-absorbing surface 108 may be effected to the cooler in control unit120. The temperature of the cooling surface 108 is less critical thanthat of the heating surface 106, but will usually be maintained in therange from 0 degrees C. to 35 degrees C. preferably being below 30degrees C. The temperature of the cooling surface may be directlycontrolled within this range, or alternatively the system may bedesigned so that the cooling temperature operates approximately withinthis range based on the total system characteristics.

The control unit 120 will further include one or more temperaturecontrollers for controlling the temperature of the heat-generatingsurface 106 and the heat-absorbing surface 106 based on the bloodtemperature and/or the body temperature. At a minimum, the control unit120 will provide for control of the temperature of the heat-generatingsurface 106 within the range set forth above, as well as for monitoringat least one of the patient blood temperature and patient bodytemperature in order to reverse the heating or cooling mode as discussedabove. In the exemplary embodiment, as described in FIG. 10, the controlscheme operates in an on-off mode, where for example hypothermicpatients are initially treated by warming the blood at a constantsurface temperature rate until a target temperature is reached. When thetarget temperature is reached, power to the heat-generating surface 106is turned off. Monitoring of the blood and/or patient body temperature,however, is maintained to assure that the patient temperature does notexceed a maximum which is above the target temperature. Should themaximum be exceeded, then the system is operated in the cooling modeuntil the excess body temperature is lowered. Usually, there will be noneed to again warm the patient, but the present system will provide forfurther cycles of warming and cooling if necessary. For initiallyhyperthermic patients, the cooling and warming modes are reversed.

It will be appreciated, for example, that the temperature controlschemes of the present invention could be substantially moresophisticated. For example, the power input to warm the patient could becontrolled based on proportional, derivative, or integral controlschemes which will typically provide for a tapering of the heat transferrate as the patient body temperature approaches the desired targetlevel. Moreover, cascade control schemes based on both patient bloodtemperature and patient body temperature could be devised. Such controlschemes, for example, could be adapted both for warming the patient andcooling the patient, with mathematical models of typical patientphysiological characteristics being taken into account in preparing thecontrol schemes. For the present, however, it is believed that a simpleoff-on control scheme with provision for reversing the heat transfermode if the target temperature is exceeded by more than a safe amountwill be sufficient.

What is claimed is:
 1. A system for restoring normal body temperature toa patient, said system comprising: an intravascular catheter having atleast one heat transfer surface; a temperature sensor; and a controlunit connectable to the temperature sensor and the catheter forselectively transferring heat to and from the at least one heat transfersurface to maintain normal body temperature.
 2. A system as in claim 1,wherein the catheter includes at least a heat-generating surface and aseparate heat-absorbing surface.
 3. A system as in claim 2, wherein theheat-generating surface comprises a resistance heater and theheat-absorbing surface comprises a metal foil wrapped around thecatheter.
 4. A system as in claim 3, wherein the resistance heatercomprises a coil and the metal foil has an exposed area of at least 2cm².
 5. A system as in claim 4, wherein the control unit comprises anelectrical current source connectable to the resistance heater and athermoelectric cooler connectable to the metal foil.
 6. A system as inclaim 1, wherein the catheter includes at least one flow lumen whichpermits flow of a heat exchange medium past the heat transfer surface,and wherein the control unit includes a heater, a cooler, and acontroller for selectively activating the heater or the cooler to heator cool the heat exchange medium and restore normal body temperature tothe patient.
 7. A system as in claim 6, wherein the heater is anelectrical resistance heater and the cooler is a thermoelectric cooler.8. A system as in claim 1, wherein the temperature sensor is on thecatheter and measures blood temperature.
 9. A system as in claim 1,wherein the temperature sensor is separately attachable to the patientto measure body temperature.
 10. A catheter for restoring normal bodytemperature to a patient by selectively transferring heat to or fromblood flow, said catheter comprising: a catheter body having a proximalend and a distal end which is insertable into a blood vessel; aheat-generating heat exchange surface near the distal end of thecatheter; and a heat-absorbing heat exchange surface near the distal endof the catheter.
 11. A catheter as in claim 10, wherein the catheterbody has a length in the range from 15 cm to 50 cm and a diameter in therange from 1 mm to 5 mm.
 12. A catheter as in claim 10, wherein theheat-generating heat transfer surface comprises an electrical resistanceheater and wherein the catheter further comprises a connector whichconnects the electrical resistance heat to an external current source.13. A catheter as in claim 10, wherein the heat-absorbing heat transfersurface comprises a metal foil wrapped around the catheter body.
 14. Acatheter as in claim 13, wherein the metal foil extends from near thedistal end to near the proximal end of the catheter body and wherein theproximal end of the foil is configured to engage an external cooler. 15.A method for restoring normal body temperature to a patient having abody temperature above or below normal body temperature, said methodcomprising: selectively introducing heat to the blood flow forhypothermic patients or removing heat from the blood flow fromhyperthermic patients; monitoring a temperature characteristic of thepatient; and selectively removing heat through the catheter from theblood flow of initially hypothermic patients if the temperaturecharacteristic indicates that the patient has or will becomehyperthermic or introducing heat through the catheter to the blood flowof initially hyperthermic patients if the temperature characteristicindicates that the patient has or will become hypothermic.
 16. A methodas in claim 15, wherein the heat is transferred via a catheter insertedinto a blood vessel selected from the group consisting of the femoralartery, the jugular artery, and the jugular vein.
 17. A method as inclaim 15, wherein the heat introducing steps comprise introducing heatat a rate between 10 W and 250 W.
 18. A method as in claim 17, whereinthe heat introducing step comprises directing current through aresistance heater near the distal end of the catheter, passingradiofrequency current from the distal end of the catheter through theblood, circulating a heated medium through a heat exchanger near thedistal end of the catheter, or directing light energy through a waveguide to the distal end of the catheter.
 19. A method as in claim 15,wherein the heat removing steps comprise removing heat at a rate between1 W and 100 W.
 20. A method as in claim 19, wherein the heat removingstep comprises (a) engaging a proximal end of the catheter against acooler in order to conductively remove heat from a distal portion of thecatheter along a heat conductive path on the catheter and to the cooleror (b) circulating a cooling fluid from the proximal end of thecatheter, through a distal portion of the catheter, and back to theproximal end.
 21. A method as in claim 15, wherein the temperaturecharacteristic monitoring step comprises monitoring at least one of bodytemperature and blood temperature.
 22. A method as in claim 15, whereinthe surface temperature of the heating surface is maintained below 42.degree. C.
 23. A method as in claim 15, wherein blood is being heated,wherein heating is stopped when the blood temperature reaches 36.9.degree. C.
 24. A method as in claim 15, wherein blood is being cooled,wherein cooling is stopped when the blood temperature drops to 36.9° C.